1. Field of the Invention
The present invention relates to a method for forming a thin film of a metal compound on a substrate through magnetron sputtering or vacuum evaporation in a stable manner and at a high rate of deposition and to a film deposition apparatus for carrying out the same.
2. Description of the Related Art
There has been widely practiced the formation, through sputtering, of thin films of metals or metal compounds, such as oxides, nitrides, fluorides, and the like. In contrast with the formation of thin films of metal, thin films of metal compounds, such as oxides, nitrides, fluorides, and the like are typically formed by one of the following methods:
(1) A reactive gas (e.g. oxygen gas, nitrogen gas, or fluorine gas) is introduced into a chamber equipped with a metal compound target (insulative) or a metal target (conductive), and a thin film is deposited through reactive sputtering activated by a radio-frequency (RF) power source.
(2) A reactive gas is introduced into a chamber equipped with a metal target, and a thin film is deposited through DC (direct current) reactive magnetron sputtering activated by a direct current (DC) power source.
The above two methods involve the following problems. The rate of deposition of a thin film is relatively low, particularly in the case of RF sputtering. The temperature of a substrate rises due to plasma, and thus sputtering at a temperature not higher than 100.degree. C. is difficult, particularly in the case of RF sputtering. In the case of DC reactive magnetron sputtering, arc discharge on a target, particularly on an uneroded portion of the target, causes the material of the target to scatter onto a substrate, and such scattering conceivably causes generation of a defect on a thin film in process of deposition.
In the case of RF magnetron sputtering, charges accumulate in an insulative thin film which is formed on an apparatus component having a grounding potential, a substrate, a substrate holder, or the like, and cause an anomalous discharge. Upon the occurrence of such an arc discharge on a certain component or the like, the material thereof scatters onto a substrate, or upon the occurrence of an arc discharge on a substrate, an arc mark remains on the substrate. Such scattering or an arc mark conceivably causes generation of a defect on a thin film in process of deposition. As the size of a substrate increases, the frequency of this phenomenon increases.
When a metal compound thin film is deposited through sputtering, an incomplete metal compound is apt to be produced due to lack of a component element of the metal compound, such as oxygen, nitrogen, or fluorine. For example, an SiO.sub.2 thin film--which is a typical oxide thin film and used as an optical film, an insulating film, a protection film, or a like film--is generally deposited by subjecting an SiO.sub.2 target (insulative) to RF magnetron sputtering activated by a radio-frequency power source or by subjecting an Si target (conductive) to DC magnetron sputtering activated by a DC power source. In this case, if the introduction of oxygen gas--which serves as a reactive gas and is introduced concurrently with Ar gas serving as a working gas for sputtering--is insufficient, the composition of a formed thin film will become SiO.sub.x (x&lt;2). In order to prevent this phenomenon, oxygen is introduced into the sputtering atmosphere in a sufficient amount for reaction. However, this causes a reduction in the deposition rate of a thin film to 1/5 to 1/10 the deposition rate of a thin film of metal.
Also, the thus-introduced reactive gas reacts with the target to produce SiO.sub.2 on the surface of the target. Charges of positive argon ions and positive oxygen ions in plasma accumulate in the produced SiO.sub.2. When the accumulation of the positive charges exceeds the dielectric limit of a SiO.sub.2 film, dielectric breakdown occurs. Alternatively, arc discharge occurs on an earth shield (anode), which is a conductive portion of a target, thereby releasing accumulated charges. This is the mechanism of an anomalous discharge occurring on a target. Such an arc discharge involves the following problems.
The material of a target scatters onto a substrate, causing generation of a defect on a thin film in process of deposition.
An arc mark remains on the surface of a target, and SiO.sub.2, an insulative substance, accumulates progressively around the arc mark, causing another anomalous discharge.
Also, in the vacuum evaporation method, a thin film of a metal compound is effectively formed through the repetition of the following steps: deposition of a metallic ultra-thin film and conversion of the metal to a metal oxide. In this case, an anomalous discharge may occur on an evaporation source, particularly on the cathode portion of an electron beam source, due to a formed reaction product (insulative substance).
Generally, the rate of deposition through sputtering is 1/2 to 1/10 the rate of deposition through vapor deposition, in which a deposition material is ion-beam-heated or resistance-heated. Therefore, sputtering is not suited for mass production.
In the method of forming a thin film through sputtering, plasma is generally utilized. Thus, collision of charged particles (ions and electrons) causes the heating of component parts of a film deposition apparatus, a substrate holder, a substrate, etc. As a result, the deposition of a film on a material having poor heat resistance, such as plastic, is difficult. This is particularly so in the case of RF magnetron sputtering which uses a radio-frequency power source.
The above-mentioned problems obstruct the application of sputtering to the formation of a compound thin film.
The applicants of the present invention formerly proposed the following methods for forming a thin film of a metal compound.
(1) A thin film of a metal compound having a desired thickness is formed by repeating the steps of: depositing on a substrate an ultra-thin film of metal, such as titanium or a like metal, through sputtering; and irradiating the ultra-thin film with an ion beam of a reactive gas, such as an oxygen gas or a like gas, so as to convert the ultra-thin film to that of a metal compound, such as titanium oxide or the like (Japanese Patent Publication No. 8-19518).
(2) A thin film of a metal compound having a desired thickness is formed by repeating the steps of: depositing on a substrate an ultra-thin film of metal through sputtering; and irradiating the ultra-thin film with plasma of a reactive gas generated by an inductive plasma source so as to convert the ultra-thin film to that of a metal compound (Japanese Patent Application Laid-Open (kokai) No. 8-176821).
However, the above-mentioned method (1) was found to involve the following problems: an ion gun involves the replacement of a consumed filament; a large number of component members, such as a filament, a screen electrode, and a suppressor electrode, are employed; the employment of a large number of component members causes contamination of a vacuum chamber; an increased screen electrode current requires a power source having a larger current capacity; and a neutralizer involves a temperature rise problem. The method (2) was found to involve the following problem: since a substrate is irradiated with charged particles (Ar ions, reactive gas ions, and electrons) in the plasma form, the charged particles damage the substrate and a thin film in process of deposition on the substrate and cause an increase in substrate temperature.